This invention relates to lithographic printing, and more particularly to a photolithographic image receptor sheet hving specific utility in silver salt diffusion transfer imaging systems.
Planographic printing plates typically contain a substrate having a hydrophilic coating thereon, the oleophilic or ink receptive image areas subsequently being produced directly on the hydrophilic surface. Various light-sensitive materials can be utilized to prepare such image areas, e.g., diazo resins, silver halide emulsions, cinnamic acid esters, etc.
One system having specific utility herein involves preparation of photographic images of silver by diffusion transfer principles, which are generally well known in the art. Typically, an imagewise exposed silver halide emulsion is contacted with a processing composition containing a developing agent to reduce the exposed silver halide to silver in the emulsion and a silver halide solvent which is capable of providing a soluble silver complex with the unexposed silver halide. An imagewise distribution of this silver complex is then transferred to a superposed silver-receptive stratum where it is reduced to metallic silver to impart to the surface thereof a silver transfer image.
Diffusion transfer has been employed in connection with photolithography by utilizing a hydrophilic material as the silver receptive stratum (to thereby provide the background of the printing plate). The silver image is then used to generate an ink-receptive printing surface on the hydrophilic background. In such a system, the hydrophilic silver-receptive stratum of the receptor sheet contains silver-precipitating nuclei disbursed in a macroscopically continuous vehicle or matrix. One such matrix having particular utility is colloidal silica, the silver being deposited imagewise on the silica surface to provide the ink-receptive image areas of the plate. The silver image areas can subsequently be chemically processed to render same oleophilic, i.e., ink-receptive.
Colloidal silica has conventionally been utilized for background areas, i.e., the hydrophilic areas of printing plates, because silica optimizes the hydrophilicity thereof, most organic materials being less hydrophilic than silica and therefore tending to produce a printing plate which may scum or accept ink in the background areas more readily than those wherein silica is utilized. U.S. Pat. No. 3,055,295 discusses the utility of silica in connection with polymers and a hardener therefor to produce printing plates. U.S. Pat. No. 3,736,872 improves thereon by utilizing self-hardening polymers therewith, and U.S. Pat. No. 3,922,441 discusses the utility of positively-charged silica in printing plate structures. A major improvement thereover is disclosed in Boston et al U.S. application Ser. No. 504,244, abandoned and refiled as Ser. No. 890,313, also abandoned, wherein it is disclosed that by utility of different silica particle sizes, an optimum of strength and imaging characteristics can be achieved without necessity of utilizing additional organic polymers in the layer.
In the foregoing constructions, attention has typically centered on improvement of the hydrophilic surface and adhesion of silver thereto, with little or no attention directed to how the hydrophilic silica layer itself is anchored to the supporting substrate or backing. I have discovered that by incorporating hereinafter defined titanium dioxide materials in an anchor layer, a dramatic increase in press life is attained over the heretofore discussed constructions due to increased adhesion of the hydrophilic silica layer to the plate backing.